This invention relates to pneumatic bias tires for off-road vehicles, and more particularly to an improvement of the bias-structured carcass composed of plural plies each containing organic fiber cords embedded in rubber in the tires of this type used under heavy load. The tires of the invention are mainly used for large size trucks, construction vehicles, industrial vehicles and the like, which travel on rough road or on road at least one part of which is inclusive of an off-road area.
The above type tires, i.e. tires having a bias-structured carcass composed of plural plies each containing organic fiber cords embedded in rubber having many merits, but also have some demerits. Particularly, a significant problem is that the number of plies constituting the carcass (hereinafter referred to as carcass ply) is increased for imparting a desirable strength matching with use conditions to the tire, which will concretely be described below. For instance, a rubberized ply composed of nylon cord of 1890 denier/two strands is mainly used at present as a carcass. When such a rubberized ply is applied to the tire for the large size truck, construction vehicle, industrial vehicle or the like, the number of carcass plies is generally 10-20 plies, while in case of the tire for superlarge size truck, the number of carcass plies frequently exceeds 40 plies. However, even when using such a large number of plies, if the conditions of use such as the running on a rough road and the like become severer, cut failure and burst failure are frequently caused.
Burst failure is roughly classified into a cut burst resulting in burst through the cut failure and a shock burst not passing through the cut failure. In general, the cut failure and cut burst are generated when the tire rides on sharp rocks, while the shock burst is generated when the tire rides on relatively obtuse rocks.
That is, as exaggeratively shown in FIG. 1, when a tire 1 rides on sharp rocks 3 projecting from a surface of a road 2, a large tensile strain T is produced in an inner surface portion 4 of the tire 1 due to the deformation thereof. When the rock 3 is relatively obtuse, if the tensile strain produced in the inner surface portion 4 exceeds a certain value, rupture starts from the cords of the carcass ply facing the inner surface of the tire at that inner surface portion and finally burst is caused together with the action of internal pressure, resulting in a shock burst. When the rock is sharp, the tensile strain T is also produced in the inner surface portion 4 of the tire 1, but a larger strain is produced in an outer surface portion 5 of the tire around the pointed end of the rock. This results in the rupture of tire beginning from the pointed end of the rock 3 or the cut failure and hence the cut burst.
In order to improve the strength of the carcass for the prevention of cut failure and burst failure, there have hitherto been proposed, a method of further increasing the number of carcass plies, a method of increasing the strength per cord and the like. However, these methods are not favorable in view of the production cost because the quantity of cords used increases considerably.